physics problem friction?

Question

A sled with a mass of 135kg is frozen to a 20 degree slope with M static (the coefficient of friction) of .4 How much additional force is required to start the sled moving down the hill?

Answer 1

F=f - Fd
F= umg cos(20) - mg sin(20)
F=mg( u cos(20) - sin(20))
F=135 x 9.8(0.4 cos(20) -sin(20))
F=44.8N